Pressure dependence of the electronic structure and spin state in Fe
1.01Se superconductors probed
by x-ray absorption and x-ray emission spectroscopy
J. M. Chen,1,*S. C. Haw,1J. M. Lee,1,3T. L. Chou,1S. A. Chen,1K. T. Lu,1Y. C. Liang,1Y. C. Lee,1N. Hiraoka,1 H. Ishii,1K. D. Tsuei,1Eugene Huang,2and T. J. Yang3
1National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan, Republic of China 2Center for General Education, Chung Chou Institute of Technology, Changhua County, Taiwan, Republic of China
3Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
(Received 25 April 2011; revised manuscript received 4 July 2011; published 9 September 2011) Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe1.01Se superconductors
up to ∼66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of∼7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe1.01Se decreases progressively with pressure up to∼10 GPa. A new prepeak at energy
of∼7113.7 eV develops for pressure above ∼13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe1.01Se using the FDMNES code agree satisfactorily. The
larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe1.01Se upon
applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe1.01Se. A discontinuous variation of the integrated absolute difference values of the Kβ emission line was
observed, originating from a phase transition of Fe1.01Se for a pressure >12 GPa. Fe1.01Se shows a small net
magnetic moment of Fe2+at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite
line Kβwas reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe1.01Se superconductors. The experimental results provide
insight into the spin state of Fe1.01Se superconductors under pressure.
DOI:10.1103/PhysRevB.84.125117 PACS number(s): 74.25.Jb, 78.70.Dm, 78.70.En
I. INTRODUCTION
The discovery of unconventional high-Tc superconduc-tors in iron-based oxypnictide compounds LaFeAsO1-xFx with a Tc of ∼26 K has sparked interest in layered FeAs
systems.1,2 Other compounds belonging to the same family,
LnFeAsO1-xFx (Ln= Ce, Pr, Nd, Sm), with Tc ∼55 K in SmFeAsO1-xFx are known.3–5 A new superconductor in an arsenic-free PbO-type β-FeSex compound with a Tcof∼8 K was reported.6 A large enhancement of T
cwas observed in a tetragonal Fe1.01Se superconductor under pressure.
Tc of Fe1.01Se increased to as much as 27 K on applying
pressure (P) 1.5 GPa and then increased to 34–37 K with pressure from 8.9 to 22 GPa.7–11 This PbO-type β-FeSe
x compound is a candidate for industrial applications, such as superconducting metal-sheathed wires.12At ambient pressure and room temperature, Fe1.01Se has a tetragonal PbO-type
structure of (P4/nmm) composed of a stack of edge-sharing FeSe4-tetrahedra layer by layer along the c axis.6The Fe1.01Se
compound exhibits planar FeSe layers similar to FeAs layers in the FeAs-pnictide superconductors. X-ray diffraction (XRD) of Fe1.01Se at varied pressure by Medvedev et al., show that
at P < 12 GPa the tetragonal form dominates but at P > 12 GPa the sample contains a mixture of tetragonal and hexagonal (NiAs-type) (P63/mmc) forms.8 A wide region of hexagonal and tetragonal phases is proposed, extending from P≈ 7–35 GPa. Above 38 GPa, Fe1.01Se transforms to
a hexagonal close-packed NiAs-type structure.8 In contrast, a phase transition from the tetragonal to an orthorhombic symmetry (Pbnm) > 12 GPa and no mixed-phase region from 11 to 33 GPa was reported by several authors.9,10,13 These results seem inconsistent. The local structure around Fe
atoms and interlayer separation in Fe-based superconductors, including iron arsenides and chalcogenide, is correlated with
Tcvariation.14 Further experimental investigation to provide direct information on the local structure around the Fe atoms in Fe1.01Se under pressure is accordingly indispensable.
The electronic states near the Fermi level in FeSe chalco-genide and FeAs pnictide are given by the Fe 3d orbitals and Se/As 4p orbitals.15–17 The interaction of Fe 3d orbitals with the neighboring Se/As orbitals in the Fe-based super-conductors affects their fundamental transport properties and physical nature.18–21 Short-range antiferromagnetic (AFM) spin fluctuations for Fe1.01Se, which were strongly enhanced
toward Tc, were observed with 77Se nuclear magnetic res-onance (NMR) spectra.22 The AFM spin fluctuation hence plays an important role in Fe-based superconductors.15,16,22,23 Because interlayer Se-Se interactions of Fe1.01Se are weak, the
application of external pressure has a profound influence on the local distortion around the Fe atoms and the hybridization between Fe 3d and interacting Se orbitals. Authors have proposed that superconductivity in Fe1.01Se is likely the result
of an intricate interplay among their structural, magnetic, and electronic properties.19,24 A comprehensive understanding of the evolution of magnetic spin states and the hybridization of the Fe 3d and the neighboring Se orbitals in Fe1.01Se
with pressure is thus of key importance for the elucidation of superconducting properties of Fe-based materials.25 How-ever, detailed information about the pressure dependence of electronic structures and spin states of Fe1.01Se is sparse.
In addition to being sensitive to variation of electronic structure, x-ray absorption spectroscopy (XAS) with chemical selectivity is widely applied to provide insight complementary
to XRD measurements in the determination of the local environment around photoabsorber atoms.26,27 We measured x-ray absorption spectra with partial fluorescence yield to obtain a high-resolution spectrum.28Fe K-edge x-ray absorp-tion spectra at high resoluabsorp-tion, particularly in the pre-edge region, provide accurate information about the electronic and local structure of Fe1.01Se under pressure. X-ray emission
spectroscopy (XES) is directly sensitive to the magnetic structure of transition metals in materials and can probe the evolution of the spin state of the metal ion as a function of pressure.29 In this work, we investigated the electronic structure, spin state, and structural transformation of iron-chalcogenide Fe1.01Se superconductors under P ∼66 GPa
on recording lifetime-broadening-suppressed x-ray absorption spectra and x-ray emission spectra.
II. EXPERIMENTS
Polycrystalline samples of Fe1.01Se (FeSe0.99) were
syn-thesized from Fe 4N and Se 4N as initial materials in a stoichiometric molar ratio 1:0.99 by a solid-state reaction.13 The reactants were weighed, mixed, ground, and pelletized in a glove box (Braun) under a purified argon atmosphere. The pellet was packed and sealed under a vacuum of
<10−4 Torr in a quartz tube and then calcined at 670 ◦C for 37 hr with intermediate grindings before quenching into a brine. The sample was characterized by refining the XRD pattern collected at the BL01C beamline at the National Synchrotron Radiation Research Center (NSRRC) using the Rietveld method. The Fe1.01Se sample contained a minor
hexagonal P63/mmc phase of the NiAs type, as reported elsewhere.7–10,13 The PbO-type tetragonal phase of Fe1.01Se
with a Tc of ∼8 K is dominant. The molar fractions of the major tetragonal phase and the secondary hexagonal phase based on Rietveld refinements are 0.89 and 0.11, respectively. A sample of Fe1.01Se as a finely grained powder was loaded
into a hole (diameter 100 μm) of a Be gasket mounted on a Mao-Bell type diamond anvil cell (culet size 250 μm). Silicone oil served as a medium to transmit pressure. The pressure in the cell was measured through the line shift of ruby luminescence with an accuracy of ∼0.1 GPa. The applied pressure was averaged at multiple points of the ruby luminescence before and after each spectral collection. All measurements were performed at room temperature.
Fe K-edge x-ray absorption and Fe Kβ x-ray emission were measured at the Taiwan inelastic x-ray scattering beam-line BL12XU at SPring-8. The undulator beam was made monochromatic with two Si(111) crystals and focused to a spot of area of∼30×30 μm2at the sample position using two Kilpatrick-Baez focusing mirrors (length 1 m). The emitted x-ray fluorescence was collected at 90◦from the incident x rays and analyzed with a Johann-type spectrometer equipped with a spherically bent Si(531) crystal (radius 1 m) and a solid-state detector arranged on a horizontal plane in the Rowland-circle geometry. The overall energy resolution, evaluated from the quasielastic scattering from the sample, had a full width of ∼0.9 eV at half maximum about the emitted photon energy of 7058 eV. The Fe K-edge energies were calibrated by measuring a simultaneous standard of Fe metal foil with the known
FIG. 1. (Color online) Fe K-edge x-ray absorption spectra, recorded with partial fluorescence yield, of polycrystalline Fe1.01Se
for pressure varied in the range 0.5–65.6 GPa. The inset displays the crystal structure of Fe1.01Se.
Fe K-edge absorption inflection point at 7112 eV and have an accuracy better than 0.05 eV. All absorption spectra are normalized to the incident beam intensity.
III. RESULTS AND DISCUSSION
Figure 1 shows Fe K-edge x-ray absorption spectra of polycrystalline Fe1.01Se recorded at P varied in the range 0.5–
65.6 GPa. The spectra were obtained in partial fluorescence yield, with the spectrometer energy fixed at the maximum of the Fe Kβ13emission line (∼7058 eV). At ambient pressure,
the Fe K-edge absorption spectrum of tetragonal Fe1.01Se
consists of a pre-edge peak A1 and two pronounced broad lines B and C on the side of greater photon energy. The general features of the Fe K-edge spectrum of tetragonal Fe1.01Se are
similar to those of LaFeAsO1−xFx samples.30,31 Because the Fe atom in Fe1.01Se is in a tetrahedral site without a center
of symmetry, the Fe 3d orbital is hybridized with Fe 4p orbitals. As the quadrupole transition Fe 1s → 3d is very weak, the observed pronounced pre-edge peak A1 in Fe K-edge x-ray absorption spectra of Fe1.01Se in Fig.1predominantly
originates from the dipole transition of a Fe 1s electron to unoccupied Fe 3d-Se 4p hybrid bands.32 Accordingly, a varying intensity of pre-edge peak A1 is thus an indicator of a varying local geometry or local distortion around the Fe atoms in Fe1.01Se under pressure. The high-energy features B and
C are dominated by dipole transitions from the Fe 1s core electron to Fe 4p unoccupied states. Feature B,∼7120 eV, is ascribed to unoccupied Fe 4p-Se 4d hybrid bands.32Feature D at greater energy is due mainly to the photoelectron multiple scattering of Fe atoms with their nearest neighbors.
The intensity of the pre-edge line A1 at energy of ∼7112.7 eV decreases progressively with P ∼10 GPa, indicating a varying local distortion around Fe atoms and a suppression of the Fe 3d-Se 4p hybridization.32 A new prepeak A2 at energy of∼7113.7 eV develops for P > 13 GPa and attains its maximum intensity at P≈ 40 GPa, indicating
formation of a new phase and in agreement with previous XRD experiments under pressure reported in the literature.8,9,11,13 After P > 40 GPa, the intensity of pre-edge peak A2 decreases slightly. Upon increasing pressure, a substantial decrease of absorption features B and C, particularly feature B, was observed, along with an energy shift. This upward shift of energy for absorption features B and C occurs because the bond lengths of both Fe-Fe and Fe-Se in Fe1.01Se decrease
upon applying pressure.9,13 The rising absorption edge of Fe
K-edge spectra of Fe1.01Se gradually shifts toward greater
energy with increasing applied pressure, probably associated with the charge transfer between Fe and Se due to the shrinking of the bond lengths of Fe-Se upon pressurization.
The inset in Fig.1displays the crystal structure of Fe1.01Se.
Based on XRD of Fe1.01Se, the interlayer Se(1)–Se(2) distance
along the c axis has greater compression than the intralayer Se(1)–Se(1) distance on applying P 8–12 GPa.9,11,13 The separation between FeSe interlayers along the c axis in Fe1.01Se
hence decreases greatly upon applying pressure. As reported by Margadonna et al., the c axis contracts by 7.3% at 7.5 GPa, whereas the basal plane contracts by 3.3% at 7.5 GPa.11This indicates a larger distortion along the stacking direction of FeSe layers in the c axis relative to the basal plane. Besides, the larger Se-Fe-Se bond angle of∼112◦in the FeSe4tetrahedra in
Fe1.01Se slightly increases with increasing pressure, whereas
the smaller Se-Fe-Se bond angle of∼105◦ decreases rapidly with P 8–12 GPa.9,11 This indicates that the distortion of FeSe4tetrahedra in Fe1.01Se increases considerably away from
regular tetrahedral shape with increasing pressure. Accord-ingly, relative to the FeSe4 tetrahedra in Fe1.01Se without a
center of symmetry at ambient pressure, the local structure of FeSe4 unit becomes a relatively centrosymmetric character
of the Fe site upon pressurization, consequently reducing the p-d hybridization. Based on polarized Fe K-edge x-ray absorption of FeSex crystals, pre-edge peak A and feature B show a larger density of states of corresponding to unoccupied Fe 3d-Se 4p and Fe 4p-Se 4d hybrid bands, respectively, along the c axis.33,34 The larger compression along the c axis, accompanied by an increased FeSe4tetrahedral distortion of
Fe1.01Se upon pressurization, suppresses the Fe 3d-Se 4p and
Fe 4p-Se 4d hybridization, particularly along the c axis, and consequently leads to a decreased intensity of pre-edge peak A and feature B.35 T
c of Fe1.01Se increased to 34–37 K on
applying P 5.5–9 GPa and then slowly decreased under P from 10 to 22 GPa.7–11The greater the suppression of Fe 3d-Se 4p hybridization upon pressurization, the higher the observed superconducting transition temperature.
With the FDMNES code,36we performed Fe K-edge x-ray absorption calculations on Fe1.01Se with different structural
symmetries, based on the structural parameters at various pressures from the literature.13In the present XAS simulation, a muffin-tin (MT) full-multiple-scattering (FMS) approach with the real energy-dependent exchange Hedin-Lundqvist potential was applied with a cluster radius R = 5 ˚A. The FMS calculations were performed using the MT potential constructed from 10% overlapped MT spheres of the specified radii. The calculated spectra were broadened using the Seah-Dench formula.36Figure2shows the simulated Fe K-edge ab-sorption spectra of Fe1.01Se with varied structural symmetries
under pressure. The simulated Fe K-edge absorption spectrum
FIG. 2. (Color online) The simulated Fe K-edge absorption spectra of Fe1.01Se for the tetragonal, orthorhombic Pbnm, and
hexagonal phases using the FDMNES code. The structural parameters of Fe1.01Se with different structural symmetries are based on the XRD
refinements in the literature (Ref.13).
of Fe1.01Se with tetragonal symmetry reproduces nicely with
the overall profile of the Fe K-edge x-ray absorption spectrum of tetragonal phase Fe1.01Se at ambient pressure in Fig.1. The
pre-edge peak A1 is shifted to higher energy with increasing pressure, and a new pre-edge peak A2 develops for P > 13 GPa, originating from a phase transition from the tetragonal structure to an orthorhombic symmetry. The experimental and calculated Fe K-edge spectra of Fe1.01Se agree satisfactorily.
In Fig.3, the evolution of the Fe Kβ x-ray emission line of Fe1.01Se as a function of P ∼66 GPa is reproduced. The Kβ
emission spectra in Fig.3are normalized to the integrated area. The Fe Kβ emission corresponds to a radiative decay of a Fe 1s core hole to a 3p level. As shown, the Fe Kβ x-ray emission spectrum is divided into a main line Kβ1,3(∼7058 eV) and a
FIG. 3. (Color online) Evolution of the Fe Kβ x-ray emission line of Fe1.01Se as a function of P ∼66 GPa. At the bottom, intensity
differences from the highest pressure emission spectrum (65.6 GPa) are shown for 0.5, 9.6, 32.5, 42.0, and 52.0 GPa. The inset displays the Kβ’䊐satellite region for 0.5, 9.6, 25.7, 42.0, and 52.0 GPa.
satellite line Kβ(∼7045 eV) due to the exchange interaction between the 3p core hole and the unfilled 3d shell in the final state of the emission. The energy separation between the satellite line Kβand the main line Kβ1,3is proportional
to the strength of the exchange interaction. The intensity of the satellite line Kβ is proportional to the net spin of the 3d shell and thus is indicative of the spin magnetic moment, as established by numerous studies on 3d transition-metal compounds.37,38
From Fig. 3, the Fe Kβ emission line shape shows a considerable variation between 0.5 and 52 GPa, with the intensity of the satellite line Kβprogressively decreasing and the energy of main emission line Kβ1,3 shifting to smaller
emission energy. The position of the Kβ1,3 line is shifted
toward lower energy by∼0.6 eV with P increased to ∼52 GPa. Above 52 GPa, the main line Kβ1,3for Fe1.01Se is narrower
and more symmetric, with the intensity of the Kβ line and the position of the Kβ1,3 line remaining nearly unchanged.
The satellite line Kβwas gradually reduced in intensity upon applying pressure and became nearly absent for P≈ 66 GPa, indicating a continuous reduction of the spin moment of Fe in Fe1.01Se superconductors.
To extract the extent of the spin magnetic moment from the
Kβ emission spectra, several approaches have been applied,
focusing on the variation of the Kβ satellite intensity or the full spectral shape or the Kβ1,3 line position.39–42 The
integrated absolute value of the difference spectra is widely applied to deduce exactly the spin magnetic moment from the Kβ emission spectrum.38,41,43–45 The derived integrated absolute difference (IAD) values correlate linearly with the spin magnetic moment in the material. The procedures for analyzing the Fe Kβ emission spectrum to obtain the IAD values involve normalizing the spectral area, shifting the spectra to the same center of mass, subtracting a reference spectrum (at 65.6 GPa, in this case) from all spectra, and integrating the absolute values of these difference spec-tra. We calculated the IAD value in the energy range of 7025–7070 eV.
Figure4shows the IAD values of Fe Kβ emission spectra of Fe1.01Se as a function of applied pressure. The IAD values
of the Fe Kβ emission show a monotonic decrease for P= 0.5–52 GPa, indicating a gradual decrease of spin magnetic moment of Fe2+ in Fe1.01Se. The IAD values of the Fe Kβ
emission line decreased rapidly with P ∼13 GPa, decreased slowly after 13 GPa, and then remained nearly unchanged after 52 GPa. The discontinuous variation of the IAD values of the
Kβ emission line with varied declining slopes arises from a
phase transition from a tetragonal structure to an orthorhombic structure for P > 12 GPa in Fe1.01Se.9,13
Figure5 shows Fe Kβ emission spectra of Fe1.01Se with P= 0.5, 40.2, and 65.6 GPa and reference emission spectra of
two iron-containing compounds with iron in the+2 oxidation state, FeS (high spin), and FeS2(low spin) at ambient pressure.
The Fe Kβ emission spectra in Fig.5were normalized, with the intensity of the main line Kβ13set to unity. The Kβintensity of
Fe1.01Se is notably less than that of FeS with the high-spin Fe2+
state. Fe1.01Se hence shows a small net magnetic moment of
Fe2+at ambient pressure, consistent with other measurements from neutron scattering and M¨ossbauer spectra for Fe-based superconductors.16,46–49The profile of the Kβline for Fe1.01Se
FIG. 4. IAD values of Fe Kβ emission spectra of Fe1.01Se as a
function of applied pressure. The dashed lines are for visual guidance.
at P= 65.6 GPa is almost coincident with that of FeS2 with
the low-spin Fe2+state. This indicates that Fe
1.01Se shows the
low-spin state of Fe2+ions upon applying P ∼66 GPa. Based on spin-lattice relaxation in77Se NMR spectra for
Fe1.01Se, the Fe spins are collinearly AFM ordered.22 For
tetragonal β-FeSe, the Fe-Fe exchange coupling between nearest-neighbor (NN) spins is ferromagnetic (FM) and that between next-nearest-neighbor (NNN) spins is AFM, because Fe-Se-Fe angles relevant to their superexchange interactions are nearer 90◦ for the former and 180◦ for the latter.16,50–52 Based on first-principles electronic structure calculations for
β-FeSe, the NN FM superexchange interactions (J1) and NNN AFM superexchange interactions (J2) are J1 = 71 meV/Fe and J2= 48 meV/Fe, respectively, at ambient pressure.15The increased FeSe4 tetrahedral distortion away from a regular
tetrahedra shape in Fe1.01Se upon pressurization reduces the
Fe 3d-Se 4p hybridization, as evident in Fig.1, and is expected to suppress the NN FM superexchange interactions mediated by Se4p orbitals.15,50The competition between the NN FM and
FIG. 5. (Color online) Fe Kβ emission spectra of Fe1.01Se for
P= 0.5, 40.2, and 65.6 GPa with reference emission spectra of FeS (high spin) and FeS2(low spin).
the NNN AFM superexchange interactions in Fe1.01Se upon
pressurization accordingly enhances spin fluctuations.22,50–52 A subtle balance of the competition between the NN FM and the NNN AFM superexchange interactions produces a collinear AFM spin arrangement in Fe1.01Se.15,16,22,50–52 The
Fe-Fe spin fluctuations in Fe1.01Se thus decrease strongly
the net magnetic moments of Fe ions, generating small net magnetic moments of Fe2+at ambient pressure.53
IV. CONCLUSION
In this work, we presented comprehensive measurements of Fe K-edge XAS and Fe Kβ XES of Fe1.01Se superconductors
to probe the evolution of electronic structure and spin state under P ∼66 GPa. The intensity of the pre-edge line at energy of∼7112.7 eV in the Fe K-edge absorption spectra of Fe1.01Se decreases progressively with P ∼10 GPa. A
new pre-edge line at energy of∼7113.7 eV develops for P > 13 GPa, corresponding to a phase transition from the tetragonal to an orthorhombic symmetry (Pbnm) and in agreement with previous XRD experiments under pressure reported in the literature.8,9,11,13Comparison of pressure-dependent x-ray absorption spectra with FMS calculations using the FDMNES code shows satisfactory agreement between experimental and calculated Fe K-edge absorption spectra of Fe1.01Se
under pressure. The larger compression along the c axis, accompanied by an increased FeSe4tetrahedral distortion of
Fe1.01Se upon pressurization, decreases the Fe 3d-Se 4p and
Fe 4p-Se 4d hybridization. Applied pressure suppresses the NN FM superexchange interaction mediated by Se 4p orbitals and enhances spin fluctuations on the Fe sites in Fe1.01Se
through the competition between NN FM and NNN AFM superexchange interactions. The position of the Kβ1,3line is
shifted toward lower energy by∼0.6 eV for pressure increased to∼66 GPa. The discontinuous variation of the IAD values of the Kβ emission line was observed to originate from the phase transformation for Fe1.01Se > 12 GPa. Fe1.01Se shows a small
net magnetic moment of Fe2+ at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. Based on the analysis by IAD methods, the variations of Kβpeak intensity and consequently the reduction of the net spin moment of Fe in Fe1.01Se upon applying pressure show a continuous
change. Fe1.01Se shows the low-spin state of Fe2+ ions upon
applying P ∼66 GPa. The experimental results provide insight into the Fe-Fe spin fluctuations and spin state of Fe1.01Se superconductors.
ACKNOWLEDGMENT
We thank the NSRRC staff for their technical support. This research is supported by the NSRRC and the National Science Council of the Republic of China under Grant No. NSC 99 2113-M-213-006.
*jmchen@nsrrc.org.tw
1Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am.
Chem. Soc. 130, 3296 (2008).
2H. Takahashi, K. Igawa, K. Arii, Y. Kamihara, M. Hirano, and
H. Hosono,Nature 453, 376 (2008).
3H.-H. Wen, G. Mu, L. Fang, H. Yang, and X. Zhu,Europhys. Lett.
82, 17009 (2008).
4T. Y. Chen, Z. Tesanovic, R. H. Liu, X. H. Chen, and C. L. Chien,
Nature 453, 1224 (2008).
5X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, and D. F. Fang,
Nature (London) 453, 761 (2008).
6F. C. Hsu, J. Y. Luo, K. W. The, T. K. Chen, T. W. Huang, P. M.
Wu, Y. C. Lee, Y. L. Huang, Y. Y. Chu, D. C. Yan, and M. K. Wu,
Proc. Nat. Acad. Sci. USA 105, 14262 (2008).
7Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, and Y. Takano,
Appl. Phys. Lett. 93, 152505 (2008).
8S. Medvedev, T. M. McQueen, I. A. Troyan, T. Palasyuk,
M. I. Eremets, R. J. Cava, S. Naghavi, F. Casper, V. Ksenofontov, G. Wortmann, and C. Felser,Nat. Mater. 8, 630 (2009).
9G. Garbarino, A. Sow, P. Lejay, A. Sulpice, P. Toulemonde,
M. Mezouar, and M. N´u˜nez-Regueiro,Europhys. Lett. 86, 27001 (2009).
10D. Braithwaite, B. Salce, G. Lapertot, F. Bourdarot, C. Marin,
D. Aoki, and M. Hanfland,J. Phys. Condens. Matter 21, 232202 (2009).
11S. Margadonna, Y. Takabayashi, Y. Ohishi, Y. Mizuguchi,
Y. Takano, T. Kagayama, T. Nakagawa, M. Takata, and K. Prassides,
Phys. Rev. B 80, 064506 (2009).
12O. Tkachenko, A. Morawski, A. J. Zaleski, P. Przyslupski, T. Dietl,
R. Diduszko, A. Presz, and K. Werner-Malento,J. Supercond. Nov. Magn. 22, 599 (2009).
13R. S. Kumar, Y. Zhang, S. Sinogeikin, Y. Xiao, S. Kumar, P. Chow,
A. L. Cornelius, and C. Chen,J. Phys. Chem. B 114, 12597 (2010).
14M. D. Lumsden and A. D. Christianson,J. Phys. Condens. Matter
22, 203203 (2010).
15F. Ma, W. Ji, J. Hu, Z.-Y. Lu, and T. Xiang,Phys. Rev. Lett. 102,
177003 (2009).
16A. Kumar, P. Kumar, U. V. Waghmare, and A. K. Sood,J. Phys.
Condens. Matter 22, 385701 (2010).
17E. Z. Kurmaev, R. G. Wilks, A. Moewes, N. A. Skorikov, Yu. A.
Izyumov, L. D. Finkelstein, R. H. Li, and X. H. Chen,Phys. Rev. B 78, 220503 (2008).
18C.-Y. Moon and H. J. Choi,Phys. Rev. Lett. 104, 057003 (2010). 19A. Subedi, L. Zhang, D. J. Singh, and M. H. Du,Phys. Rev. B 78,
134514 (2009).
20J. Zhao, Q. Huang, C. de la Cruz, S. Li, J. W. Lynn, Y. Chen, M. A.
Green, G. F. Chen, G. Li, Z. Li, J. L. Luo, N. L. Wang, and P. Dai,
Nat. Mater. 7, 953 (2008).
21T. M. McQueen, M. Regulacio, A. J. Williams, Q. Huang, J. W.
Lynn, Y. S. Hor, D. V. West, M. A. Green, and R. J. Cava,Phys. Rev. B 78, 024521 (2008).
22T. Imai, K. Ahilan, F. L. Ning, T. M. McQueen, and R. J. Cava,
Phys. Rev. Lett. 102, 177005 (2009).
23T. Yildirim,Phys. Rev. Lett. 101, 057010 (2008).
24K.-W. Lee, V. Pardo, and W. E. Pickett,Phys. Rev. B 78, 174502
25R. A. Jishi and H. M. Alyahyaei,New J. Phys. 11, 083030 (2009). 26J. M. Chen, T. L. Chou, J. M. Lee, S. A. Chen, T. S. Chan, C. K.
Chen, K. T. Lu, H.-S. Sheu, C. M. Lin, N. Hiraoka, H. Ishii, and T. J. Yang,,Phys. Rev. B 79, 165110 (2009).
27F. Bridges, C. H. Booth, M. Anderson, G. H. Kwei, J. J. Neumeier,
J. Snyder, J. Mitchell, J. S. Gardner, and E. Brosha,Phys. Rev. B 63, 214405 (2001).
28J.-P. Rueff, L. Journel, P.-E. Petit, and F. Farges,Phys. Rev. B 69,
235107 (2004).
29J.-P. Rueff, C.-C. Kao, V. V. Struzhkin, J. Badro, J. Shu, R. J.
Hemley, and H. K. Mao,Phys. Rev. Lett. 82, 3284 (1999).
30N. Kaurav, Y. T. Chung, Y. K. Kuo, R. S. Liu, T. S. Chan, J. M.
Chen, J.-F. Lee, H.-S. Sheu, X. L. Wang, S. X. Dou, S. I. Lee, Y. G. Shi, A. A. Belik, K. Yamaura, and E. Takayama-Muromachi,Appl. Phys. Lett. 94, 192507 (2009).
31A. Ignatov, C. L. Zhang, M. Vannucci, M. Croft, T. A. Tyson,
D. Kwok, Z. Qin, and S.-W. Cheong e-print arXiv:0808.2134
(unpublished).
32F. de Groot, G. Vank´o, and P. Glatzel,J. Phys. Condens. Matter 21,
104207 (2009).
33B. Joseph, A. Iadecola, L. Simonelli, Y. Mizuguchi, Y. Takano,
T. Mizokawa, and N. L. Saini,J. Phys. Condens. Matter 22, 485702 (2010).
34B. C. Chang, Y. B. You, T. J. Shiu, M. F. Tai, H. C. Ku, Y. Y. Hsu,
L. Y. Jang, J. F. Lee, Z. Wei, K. Q. Ruan, and X. G. Li,Phys. Rev. B 80, 165108 (2009).
35H. Yamaoka, I. Jarrige, A. Ikeda-Ohno, S. Tsutsui, J.-F. Lin,
N. Takeshita, K. Miyazawa, A. Iyo, H. Kito, H. Eisaki, N. Hiraoka, H. Ishii, and K.-D. Tsuei,Phys. Rev. B 82, 125123 (2010).
36Y. Joly,Phys. Rev. B 63, 125120 (2001).
37H. Yamaoka, N. Tsujii, H. Oohashi, D. Nomoto, I. Jarrige,
K. Takahiro, K. Ozaki, K. Kawatsura, and Y. Takahashi,Phys. Rev. B 77, 115201 (2008).
38A. Mattila, T. Pylkk¨anen, J.-P. Rueff, S. Huotari, G. Vank´o,
M. Hanfland, M. Lehtinen, and K. H¨am¨al¨ainen,J. Phys. Condens. Matter 19, 386206 (2007).
39J. P. Rueff, M. Krisch, Y. Q. Cai, A. Kaprolat, M. Hanfland,
M. Lorenzen, C. Masciovecchio, R. Verbeni, and F. Sette, Phys. Rev. B 60, 14510 (1999).
40J. Badro, J.-P. Rueff, G. Vank´o, G. Monaco, G. Fiquet, and F. Guyot,
Science 305, 383 (2004).
41J.-P. Rueff, A. Shukla, A. Kaprolat, M. Krisch, M. Lorenzen,
F. Sette, and R. Verbeni,Phys. Rev. B 63, 132409 (2001).
42P. Glatzel and U. Bergmann, Coord. Chem. Rev. 249, 65
(2005).
43G. Vank´o, T. Neisius, G. Moln´ar, F. Renz, S. K´arp´ati, A. Shukla,
and F. M. F. de Groot,J. Phys. Chem. B 110, 11647 (2006).
44G. Vank´o, J.-P. Rueff, A. Mattila, Z. N´emeth, and A. Shukla,Phys.
Rev. B 73, 024424 (2006).
45R. Lengsdorf, J.-P. Rueff, G. Vank´o, T. Lorenz, L. H. Tjeng, and
M. M. Abd-Elmeguid,Phys. Rev. B 75, 180401 (2007).
46Y. Qiu, W. Bao, Q. Huang, T. Yildirim, J. M. Simmons, M. A.
Green, J. W. Lynn, Y. C. Gasparovic, J. Li, T. Wu, G. Wu, and X. H. Chen,Phys. Rev. Lett. 101, 257002 (2008).
47C. de la Cruz, Q. Huang, J. W. Lynn, J. Li, W. Ratcliff II, J. L.
Zarestky, H. A. Mook, G. F. Chen, J. L. Luo, N. L. Wang, and P. Dai,Nature 453, 899 (2008).
48A. Błachowski, K. Ruebenbauer, J. ´Zukrowski, J. Przewo´znik,
K. Wojciechowski, and Z. M. Stadnik,J. Alloys Compd. 494, 1 (2010).
49Q. Huang, Y. Qiu, W. Bao, M. A. Green, J. W. Lynn, Y. C.
Gasparovic, T. Wu, G. Wu, and X. H. Chen,Phys. Rev. Lett. 101, 257003 (2008).
50Q. Si and E. Abrahams, Phys. Rev. Lett. 101, 076401
(2008).
51P. Kumar, A. Kumar, S. Saha, D. V. S. Muthu, J. Prakash, S. Patnaik,
U. V. Waghmare, A. K. Ganguli, and A. K. Sood,Solid State Comm. 150, 557 (2010).
52D.-X. Yao and E. W. Carlson, Phys. Rev. B 78, 052507
(2008).
53Z.-Y. Lu, F. Ma, and T. Xiang, J. Phys. Chem. Solids, 72, 319
